Method and device for transmitting information over a communication channel with variable impedance

ABSTRACT

A method includes digital/analog conversion of a digital signal modulated by information to provide a modulated initial analog signal having a crest factor greater than one, and amplification of the initial analog signal to provide an amplified modulated signal. A modulated channel analog signal derived from the modulated amplified analog signal is transmitted over a communications channel, with impedance of the communications channel varying during the transmission. The method further includes at least one determination during the transmission of a peak-clipping rate of the amplified analog signal over at least one time interval, and an adjustment of a level of the initial analog signal as a function of the determined peak-clipping rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/961,985, filed on Dec. 8, 2015, which claims priority to FranceApplication No. 1555084, filed on Jun. 4, 2015, all of which are herebyincorporated herein by reference.

TECHNICAL FIELD

Implementations and embodiments of the invention relate to transmissionof information over a communications channel, and in particular, whenthe communications channel is an electrical line and the transmission ofinformation is based on power line communications (PLC). Moreparticularly, processing of such a signal in transmission mode isimproved when an impedance of the transmission channel seen by thetransmitter drops.

Implementations and embodiments are compatible with the differentstandards governing power line communications, in particular, but notexclusively, PLC-G3, PRIME (PoweRline Intelligent Metering Evolution)standards or even the IEEE 1901-2 standard.

BACKGROUND

An aim of power line communications technology is to transmit digitaldata by exploiting the existing infrastructure of the electricalnetwork. In particular, it is possible to remotely read electricalmeters, allow for exchanges between electric vehicles and rechargingterminals, or even allows for management and control of energy networks(smart grid).

Power line communications (PLC) technology notably incorporatesnarrowband power line communications (N-PLC) which is generally definedas a communications over an electrical line operating at transmissionfrequencies up to 500 KHz. The N-PLC communications thus generally usesfrequency bands notably defined by the European Committee forElectrotechnical Standardization (CENELEC) or by the FederalCommunications Commission (FCC).

Thus, to consider the CENELEC A frequency band (3-95 kHz), thetransmission frequencies are situated between 42 and 89 KHz in the PRIMEstandard, whereas they are situated between 35 and 91 KHz for the PLC-G3standard.

The signals used in PLC communications are signals modulated accordingto a multicarrier modulation, for example, a quadrature modulation onorthogonal carriers (Orthogonal Frequency Division Multiplexingmodulation, or OFDM modulation), but using only a subset of carriers outof a larger set of available carriers.

Thus, for example, to consider the CENELEC A frequency band, the size ofthe inverse Fourier transform and of the direct Fourier transform isequal to 512, whereas only 97 sub-carriers (the sub-carriers 86 to 182)are used for the transmission in the PRIME standard.

To consider the CENELEC A frequency band, the size of the inverseFourier transform and of the direct Fourier transform is equal to 256while only 36 sub-carriers (the sub-carriers 23 to 58) are used in thePLC-G3 standard.

The signals used in PLC communications and modulated according to OFDMmodulation exhibit a crest factor greater than one and generally higher.The crest factor of a signal, commonly called PAPR (Peak-to-AveragePower Ratio), is a characteristic measurement of this signal. It is theratio between the absolute value of the maximum amplitude of the peaksof the signal and the effective signal value. It is equal to one for aconstant signal, and greater than one as soon as the signal exhibitspeaks.

In PLC communications, the impedance of the communications channel (theelectrical line) seen by the transmitter can vary during communicationsand can drop when a user connects any device such as, for example, ahairdryer or a washing machine.

Typically, a resistive impedance of 2 Ohms seen by the transmitterserves as a reference for determining the maximum output power of thetransmitter. Now, depending on the number of devices connected to theelectrical line, this impedance seen by the transmitter may be less than2 Ohms, or even lower.

Also, when the transmitter transmits a signal in a line having animpedance less than 2 Ohms, the power amplifier of the transmitter willenter into saturation in terms of current. The amplifier then entersinto a current limiting mode in which it clips the current peaksexceeding the authorized maximum current. The result thereof is then adistortion of the signal and a generation of noise harmonics, orharmonic interferers.

Since the transmission frequencies of the signal are situated between 42and 89 KHz in the PRIME standard and between 35 and 91 KHz for thePLC-G3 standard, the second harmonics are situated between 70 KHz and180 KHz.

Consequently, some of these harmonics interfere with the upper part ofthe useful frequency band of the signal. Furthermore, these harmonicsprovoke interferences outside of the useful band of the signal which candisrupt other equipment.

Moreover, when the transmitter has to satisfy the requirements of theEN50065-1 standard, which is the case for transmissions according to thePRIME and PLC-G3 standards, the level of the output signal of thetransmitter is measured with a peak detector over a pass band of 200 Hzand no part of the spectrum of the transmitted signal must exceed 120dBμV.

SUMMARY

An object is to take into account a drop in impedance while limitingdistortion of a transmitted signal to an acceptable level with respectto the intended application. There is a correlation between thepeak-clipping rate of the signal and the distortion level which resultstherefrom, and the peak-clipping rate of the signal may be measured andthe signal level may be adjusted as a function of the value of thispeak-clipping rate.

According to one aspect, a method for transmitting information over acommunications channel comprises a digital/analog conversion of adigital signal modulated by the information so as to obtain a modulatedinitial analog signal having a crest factor greater than one, anamplification of the initial analog signal so as to obtain an amplifiedmodulated signal and a transmission over the communications channel of amodulated channel analog signal derived from the modulated amplifiedanalog signal.

When the impedance of the communications channel is likely to varyduring the transmission, the amplified signal may be clipped when theimpedance is below a limit value, for example, 2 Ohms as seen by thetransmitter. The method may then further comprise at least onedetermination during the transmission of a peak-clipping rate of theamplified signal over at least one time interval, and an adjustment ofthe level of the initial analog signal as a function of the determinedpeak-clipping rate.

Adjustment of the level of the initial analog signal may comprise asignal adjustment performed directly in analog mode on the initialanalog signal or else indirectly on the initial analog signal by acting,for example, in digital mode directly or indirectly on the level of themodulated digital signal upstream of the digital/analog conversion.

Thus, by reducing the level of the signal at the input of the amplifier,it may be possible to reduce the peak-clipping rate and consequently theinterferences caused by the distortion since the latter is also reduced.The peak-clipping rate is, for example, the number of peaks clipped ofthe signal during the time interval divided by the length of the timeinterval.

Adjustment of the level of the initial analog signal may comprise acomparison of the determined peak-clipping rate with a threshold and alowering of the level of the initial analog signal if the peak-clippingrate is above the threshold. A person skilled in the art will be able tochoose the value of this threshold as a function of the level ofdistortion acceptable in the application considered.

Reduction of the level of the signal reduces the signal-to-noise ratio.Consequently, the optimal operating conditions may be obtained when thelevel of interference due to the distortion is substantially equal tothe noise. A person skilled in the art will be able to determine thevalue of the threshold to approximate, or even achieve, these optimaloperating conditions. That said, as a non-limiting example, fortransmissions according to the PRIME and PLC-G3 standards, a thresholdequal to 0.1% of the total number of peaks of the time interval may beconsidered to lead to an acceptable level of distortion even in lownoise conditions.

Generally, a number of successive determinations of the peak-clippingrate may advantageously be provided during successive time intervals,for example, when the information is transmitted during successiveframes. In this case, during the current time interval, a determinationmay be made on adjustment of the level of the initial analog signal tobe applied to the initial analog signal during the next time interval.

At the start of the transmission, the initial analog signal has anominal level, and if, during a current time interval for which thelevel of the initial analog signal is below the nominal level, thedetermined peak-clipping rate is below the threshold, the adjustment ofthe level of the initial analog signal to be applied during the nexttime interval may comprise an increase in the level of the initialanalog signal but without exceeding the nominal level.

When the information is transmitted by frame, each time interval may be,for example, the duration of a frame. The modulated signals may bemodulated according to an OFDM modulation. In applications of the PLCtype, the transmission channel may be an electrical line and the channelanalog signal may be a signal conveyed by power line communications.

According to another aspect, a device for transmitting informationcomprises an input for receiving a digital signal modulated by theinformation, an output to be coupled to a communications channel todeliver a modulated channel analog signal, and processing means or aprocessor connected between the input and the output and configured togenerate the modulated channel analog signal from the modulated digitalsignal.

The processor may comprise a digital/analog conversion stage configuredto perform a digital/analog conversion of the modulated digital signaland deliver a modulated initial analog signal having a crest factorgreater than one, and an amplifier stage configured to perform anamplification of the initial analog signal and deliver an amplifiedmodulated signal.

Impedance of the communications channel may likely vary during thetransmission, and the amplifier stage may be configured to clip theamplified signal when the impedance is below a limit value. Theprocessor may further comprise a control module configured to perform atleast one determination during the transmission of a peak-clipping rateof the amplified signal over at least one time interval and anadjustment of the level of the initial analog signal as a function ofthe determined peak-clipping rate.

As indicated above, adjustment of the level of the initial analog signalmay be direct or indirect. Thus, the control module may be configured todirectly adjust the level of the initial analog signal or else to adjustit indirectly by adjusting, for example, the level of the modulateddigital signal.

The control module may comprise a comparator configured to perform acomparison of the determined peak-clipping rate with a threshold, and anadjustment means or adjustment circuitry configured to lower the levelof the initial analog signal if the peak-clipping rate is above thethreshold.

The control module may be configured to perform a number of successivedeterminations of the peak-clipping rate during successive timeintervals, and to perform, during a current time interval, adetermination of the adjustment of the level of the initial analogsignal to be applied to the initial analog signal during the next timeinterval.

At the start of the transmission, the initial analog signal has anominal level, and, if during a current time interval for which thelevel of the initial analog signal is below the nominal level, thedetermined peak-clipping rate is below the threshold, the adjustmentcircuitry may be configured to, during the next time interval, increasethe level of the initial analog signal but without exceeding the nominallevel.

The digital/analog conversion stage may be a variable gain stage and theadjustment circuitry may be configured to reduce or increase the gain soas to thus adjust the level of the initial analog signal.

According to yet another aspect, a transmitter comprises a device fortransmitting information as defined above, and pre-processing means or apre-processor configured to receive the information and to generate thedigital signal modulated by the information.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent onexamining the detailed description of implementations and embodimentsthat are in no way limiting, and the attached drawings in which:

FIGS. 1 to 9 schematically illustrate different implementations andembodiments of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The implementations and embodiments which will now be described in thecontext of transmission of information by power line communications(PLC), although they are not limited to this type of application.

Throughout the following, each time the PLC-G3 or PRIME standards arecited by way of non-limiting examples, it will be assumed that it is theCENELEC A frequency band (3-95 kHz) that is being considered.

Reference is now made to FIG. 1 to schematically illustrate an exemplarytransmitter 1 capable of transmitting a useful analog signal, or achannel analog signal SU over a communications channel by power linecommunications. The illustrated communications channel is an electricalline.

The transmission chain of the transmitter comprises pre-processing meansor a pre-processor MPTR receiving the binary data, or information, to betransmitted from source coding means or a source coder, for example, andwhich are configured to generate a digital signal SN modulated by theinformation according to an OFDM modulation.

As a non-limiting example, the pre-processor MPTR as illustrated in FIG.1 comprises an encoder ENC, for example, a convolution encoder.Interleaving means or an interleaver INTL is connected to the output ofthe encoder and is followed by mapping means or mapping circuitry whichtransforms the bits into symbols according to a transformation schemedependent on the type of modulation used, for example, a BPSK typemodulation, or more generally, a QAM modulation.

Each symbol contains modulation coefficients associated with carrierswhich will be modulated accordingly. The symbols are delivered as inputfor MTFI means or circuitry to perform an inverse fast Fourier transform(IFFT) operation.

By referring more particularly to FIG. 2, the modulated carriers form asubset SNS of carriers out of an available set ENS of carriers (a setwhich corresponds to the size of the inverse Fourier transform).

Thus, in the PLC-G3 standard, the size of the inverse Fourier transformis equal to 256 whereas the modulated carriers of the subset SNS liebetween the ranks 23 and 58. This corresponds to a frequency band F1-F2lying between 35 and 91 KHz. The sampling frequency is equal to 400 KHzleading to a spacing between the carriers equal to 1.5625 KHz. Thisrenders the frequencies orthogonal (OFDM modulation).

In the PRIME standard, the size of the inverse Fourier transform isequal to 512 while the number of carriers of the subset SNS is equal to97. This provides, for the useful signal, a frequency band extendingbetween 42 and 89 KHz. The modulation coefficients associated with theunused carriers are equal to zero.

The OFDM signal in the time domain is generated as an output from theMTFI circuitry, and MCP circuitry adds to each OFDM symbol in the timedomain a cyclical prefix which is a copy at the head of the OFDM symbolof a certain number of samples situated at the end of this symbol.

Referring once again to FIG. 1, the digital signal SN, modulated by theinformation according to an OFDM modulation, and generated by thepre-processor MPTR, is delivered to the input BE of a device 10 fortransmitting information over the electrical line LE. For this, thedevice 10 comprises a processor MTR connected to the output terminal BScoupled to the electrical line LE. The processor MTR will generate thechannel analog signal SU from the digital signal SN.

More specifically, the modulated digital signal SN is converted in adigital/analog conversion stage ECNA, into an analog signal, here calledan initial analog signal, SM, which is consequently also modulated.

The initial analog signal SAI is then processed in a stage ETA, commonlyreferred to by those skilled in the art an analog front end. The initialanalog signal SAI undergoes a power amplification before beingtransmitted in the form of the modulated channel analog signal SU, overthe electrical line LE.

In addition to the circuitry which has just been described, theprocessor MTR further comprises a control module MCTL configured toperform at least one determination, during the transmission of theinformation, of a peak-clipping rate of the amplified signal within theETA stage, over at least one time interval, for example, a transmissionframe. An adjustment of the level of the initial analog signal SAI isthen performed as a function of the determined peak-clipping rate.

More specifically, as illustrated in more detail in FIG. 3, thedigital/analog conversion stage ECNA comprises an actual digital/analogconverter CNA followed by a variable gain amplifier PMP.

The stage ETA, for its part, notably comprises a power amplifier PAreceiving the initial analog signal SAI and delivering an amplifiedanalog signal SAP.

When the power amplifier PA enters into saturation, because of the dropin the impedance (seen by the transmitter) of the electrical line LEbelow a limit value, for example 2 Ohms, the amplified signal is clippedand an IRQ logic signal is transmitted. The IRQ logic signal remains,for example, in the high state as long as the power amplifier is insaturation and then drops back to the low state when the saturationstate is finished. Thus, a pulse of the IRQ signal is representative ofa clipped peak of the amplified signal SAP.

The control module MCTL comprises, for example, a computation means orcircuitry MCL configured to determine the peak-clipping rate of thesignal over a given time interval, for example, an informationtransmission frame, from the number of pulses of the IRQ signal over thetime interval, for example.

The control module MCTL moreover comprises a comparator CMP configuredto compare the duly computed peak-clipping rate TCR with a threshold TH.Based on the result of this comparison, an adjustment means or circuitryMAJ delivers a control signal SCTRL so as to adjust the level of theinitial analog signal SAI, that is, to the signal at the input of thepower amplifier PA. Typically, this level is lowered when thepeak-clipping rate TCR is above the threshold TH.

The computation circuitry and/or the adjustment circuitry can beproduced, for example, by logic circuits and/or by software within amicrocontroller. In the example described here, the signal SCTRL acts onthe variable gain amplifier PMP to modify its gain, for example, bydecrementing or by incrementing the gain by a value ΔG expressed in dB.

As a variation, it would also be possible to perform the adjustment ofthe signal by acting, for example, on the level of the digital signal SNdelivered to the digital/analog converter CNA.

FIG. 4 illustrates, in the top part, an exemplary amplified modulatedanalog signal SAP delivered as output from the power amplifier PA, andmore particularly in the present case, the change of the differentialcurrent delivered by this power amplifier during a transmission frame.This signal comprises peaks but none of these current peaks exceeds thelimit values IL⁺ and IL⁻ beyond which the amplifier will enter intosaturation.

Consequently, the IRQ signal, represented in the bottom part of Figure4, remains constantly in its low state L. In this case, thepeak-clipping rate of the signal SAP is zero and there is no distortionof the signal transmitted over the electrical line.

FIG. 5 schematically shows a configuration in which the amplified signalSAP comprises only a few clipped peaks. That is, peaks exhibiting anacceptable peak-clipping rate which leads to an acceptable distortionlevel. In this example, only two peaks PK1 and PK2 are clipped, whichresults in two pulses of the IRQ signal during the transmission frameTR.

The peak-clipping rate is then, for example, computed by counting thenumber of pulses of the IRQ signal and by dividing this number of pulsesby the frame length. This peak-clipping rate can then be converted intoa percentage of the total number of peaks of the signal SAP during thetransmission frame TR.

Typically, for a PLC application, a peak-clipping rate TCR less than orequal a threshold of 0.1% is an acceptable rate. This rate leads to anacceptable distortion of the signal.

FIG. 6, on the other hand, shows a case of a signal SAP exhibiting apeak-clipping rate above the threshold. Thus, by way of example, thesignal SAP here comprises five clipped peaks, which leads to five pulsesof the IRQ signal during the frame.

Reference is now made more particularly to FIGS. 7 to 9 to illustrateexemplary implementations of the method. In FIG. 7, it is assumed thatthe information is transmitted during successive frames, not necessarilyevenly spaced in time.

During the current frame TR_(i), the peak-clipping rate TCR isdetermined (step 60). This peak-clipping rate TCR is then compared tothe threshold TH (step 61).

If the peak-clipping rate TCR is not above the threshold TH, then, ingeneral, there is a transition to the next frame without reducing thelevel of the initial analog signal. However, as will be seen in moredetail below, it is possible in certain cases that in presence of apeak-clipping rate TCR below the threshold TH, the level of the analogsignal SAI is incremented for the next frame but without exceeding anominal level.

If, however, the peak-clipping rate TCR is above the threshold TH, thenthe level of the signal SAI is reduced (step 62). This is typically doneby lowering the gain G of the amplifier PMP. The reduced gain is thenapplied for the next frame TR_(i+1).

Reference is now made more particularly to FIGS. 8 and 9 to illustratean exemplary adjustment of the level of the analog signal SAI. It isassumed that the gain of G of the amplifier PMP has a nominal value G0corresponding to an absence of peak-clipping or to a peak-clipping ratebelow the threshold. This threshold is chosen to be equal to 0.1% of thetotal number of peaks during a frame TR duration.

It is assumed that the gain G0 is applied during the first frame TR1.During this first frame, the peak-clipping rate TCR1 is determined (step70) and is assumed equal to 0.05%. As this peak-clipping rate TCR1 isbelow the threshold TH (step 71), the adjustment circuitry MAJ does notmodify the value of the gain G and keeps it at its nominal value G0(step 72). This nominal value G0 will be applied during the next frameTR2.

During this next frame TR2, the peak-clipping rate TCR2 is once againdetermined (step 74) and it is assumed equal to 0.2%. Since thispeak-clipping rate TCR2 is above the threshold TH (step 75), a new gainvalue G is computed, namely a value G1=G0−ΔG, where ΔG is the gainincrement which will be subtracted from the nominal value G0 (step 76).This new gain G1 will be applied during the third frame TR3.

During this third frame TR3, the peak-clipping rate TCR3 is computed(step 78) and it is assumed equal to 0.3%. As this rate TCR3 is stillabove the threshold TH (step 79), a new gain value G2 will be determinedby again subtracting the gain increment ΔG from the preceding gain G1(step 80). This new gain G2 will be applied during the next frame TR4.

During this next frame TR4, the new peak-clipping rate TCR4 isdetermined (step 82) and this time it is equal to 0.1%. As thispeak-clipping rate TCR4 corresponds to the threshold TH (step 83), theadjustment circuitry MAJ does not modify the value of the gain G andleaves it equal to the value G2 (step 84). This gain G2 will then beapplied during the next frame TR5 (step 85).

During the frame TR5, the peak-clipping rate TCR is again determined(step 86) and this time it is equal to 0.003%. As the peak-clipping rateTCR5 is below the threshold TH (step 87), and the value of the gain isbelow its nominal value G0, a new gain G is determined in step 88 so asto be able to be applied for the next frame TR6.

This new gain is obtained by incrementing the preceding gain G2 by thegain increment ΔG, which then again provides the value G1 for the gainG. This value G1 is applied for the next frame TR6 (step 89).

During this next frame TR6, the peak-clipping rate TCR6 is computed andit is equal to 0.05% (step 90). As this peak-clipping rate TCR6 is belowthe threshold TH (step 91) and the value of the gain G1 is again belowthe nominal value G0, the adjustment circuitry MAJ will then confer onthe gain G its nominal value G0 by incrementing the gain G1 by the gainincrement ΔG (step 92).

The nominal gain G0 will then be applied to the next frame TR7 (step93). The method then continues for the next frames.

In the above, it was assumed that the gain increment ΔG was constant.This increment ΔG can be computed as a function of the differencebetween the measured peak-clipping rate and the threshold TH, to speedup the response time to a significant change of impedance. In this case,ΔG can vary from one frame to another.

What is claimed is:
 1. A method comprising: amplifying an analog signalwith a power amplifier to produce an amplified analog signal;transmitting over a communication channel frames of information based onthe amplified analog signal; monitoring an output current of the poweramplifier; determining a current-peak clipping rate of the monitoredoutput current; comparing the current-peak clipping rate with acurrent-peak clipping rate threshold; and modifying an amplitude of theanalog signal based on comparing the current-peak clipping rate with thecurrent-peak clipping rate threshold.
 2. The method of claim 1, furthercomprising: comparing the monitored output current of the poweramplifier with a peak current threshold; and generating a first signalbased on comparing the monitored output current of the power amplifierwith the peak current threshold, wherein the determining thecurrent-peak clipping rate is based on the first signal.
 3. The methodof claim 2, wherein determining the current-peak clipping rate is basedon a number of pulses of the first signal over a time interval.
 4. Themethod of claim 3, wherein the time interval corresponds to duration ofa frame of information of the frames of information.
 5. The method ofclaim 2, wherein the first signal remains in a first state when themonitored output current is above the peak current threshold and remainsin a second state different than the first state when the monitoredoutput current is below the peak current threshold.
 6. The method ofclaim 5, wherein the first state is a high state and the second state isa low state.
 7. The method of claim 1, wherein modifying the amplitudeof the analog signal comprises adjusting a gain of a variable gainamplifier generating the analog signal.
 8. The method of claim 7,wherein adjusting the gain of the variable gain amplifier comprisesmodifying the gain of the variable gain amplifier by a gain step.
 9. Themethod of claim 8, wherein adjusting the gain of the variable gainamplifier further comprises: increasing the gain of the variable gainamplifier by the gain step when the current-peak clipping rate is belowthe current-peak clipping rate threshold; and decreasing the gain of thevariable gain amplifier by the gain step when the current-peak clippingrate is above the current-peak clipping rate threshold.
 10. The methodof claim 1, further comprising generating the analog signal based on adigital signal with a digital to analog converter, wherein modifying theamplitude of the analog signal comprises adjusting a level of thedigital signal.
 11. The method of claim 1, wherein the communicationchannel comprises a power line communication (PLC) channel.
 12. Atransmitter comprising: an analog front end comprising a power amplifiercoupled to an output terminal, the output terminal configured to becoupled to a power line communication (PLC) channel; a digital to analogconversion stage having an input coupled to an input terminal, thedigital to analog conversion stage comprising a digital to analogconverter having an input coupled to the input terminal, and a variablegain amplifier having an input coupled to an output of the digital toanalog converter, and an output coupled to an input of the analog frontend; and a control module configured to receive a logic signal from theanalog front end, determine a current-peak clipping rate based on thelogic signal, compare the current-peak clipping rate of the poweramplifier with a current-peak clipping rate threshold, and modify a gainof the variable gain amplifier based on comparing the current-peakclipping rate with the current-peak clipping rate threshold.
 13. Thetransmitter of claim 12, wherein the analog front end is configured tocompare an output current of the power amplifier with a peak currentthreshold; and generate the logic signal based on comparing the outputcurrent of the power amplifier with the peak current threshold.
 14. Thetransmitter of claim 12, wherein determining the current-peak clippingrate is based on a number of pulses of the logic signal over a timeinterval.
 15. The transmitter of claim 12, wherein modifying the gain ofthe variable gain amplifier comprises modifying the gain of the variablegain amplifier by a gain step.
 16. The transmitter of claim 15, whereinmodifying the gain of the variable gain amplifier comprises: increasingthe gain of the variable gain amplifier by the gain step when thecurrent-peak clipping rate is below the current-peak clipping ratethreshold; and decreasing the gain of the variable gain amplifier by thegain step when the current-peak clipping rate is above the current-peakclipping rate threshold.
 17. A method for transmitting information overa power line communication (PLC) channel, the method comprising:generating an analog signal with a variable gain amplifier coupled to apower amplifier; amplifying the analog signal with the power amplifierto produce an amplified analog signal; transmitting over the PLC channelframes of information based on the amplified analog signal; monitoringan output current of the power amplifier; determining a current-peakclipping rate of the monitored output current; comparing thecurrent-peak clipping rate with a current-peak clipping rate threshold;and modifying a gain of the variable gain amplifier based on comparingthe current-peak clipping rate with the current-peak clipping ratethreshold.
 18. The method of claim 17, further comprising: performing aplurality of successive determinations of the current-peak clipping rateduring successive time intervals; and determining the gain of thevariable gain amplifier to be adjusted on a next time interval based ona determination of the current-peak clipping rate during a previous timeinterval.
 19. The method of claim 17, wherein modifying the gain of thevariable gain amplifier comprises modifying the gain of the variablegain amplifier by a gain step.
 20. The method of claim 19, whereinmodifying the gain of the variable gain amplifier comprises: increasingthe gain of the variable gain amplifier by the gain step when thecurrent-peak clipping rate is below the current-peak clipping ratethreshold; and decreasing the gain of the variable gain amplifier by thegain step when the current-peak clipping rate is above the current-peakclipping rate threshold.